Carrier wave signaling system



Aug. 2E, E94.

W. A. PHELPs y CARRIER WAVE SIGNALING SYSTEM 4 Filed Aug. 24, 1943 4Sheets-Sheet l El EN W DU H /NVENTOR W A. PHELPS ATTORNEY ug- 20, 1946w. A. PHELPs CARRIER WAVE SIGNALING SYSTEM Filed Aug. 24, 1943 J4Sheets-Sheet 2 ll my H l e me n n d4 fno nl Iv am. MC

` /NVE/vrof? W A. PHE'LPS i ATTORNEY ug. 20, 1946. W. A. PHELPS CARRIERwAvR SIGNALING SYSTEM Filed Aug. 24, 1945 4 Sheets-Sheet 3 m. @Pi

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Filed Aug. 24, 1945 4 Sheets-Sheet l DE BELOW MILL/WATT INPUT LEVEL nrs.

FPEllUE/VCYl DEV/A TION BETWEEN H E OSC/LLAVORJ CYCLES PER SECONDmiam/VFY Patented Ang. 20, i946 CARRIER WAVE SIGNALING SYSTEM Walter A.Phelps, Madison, N. J., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationAugust 24, 1943, Serial N0. 499,773

The invention relates to carrier wave signaling systems and particularlyto carrier telegraph systems.

The invention is particularly applicable to, although not limited to,such a system including as a link therein, a multichannelvoice-frequency carrier telegraph system utilizing in each channel twotones of voice frequency slightly separated in the frequency spectrum,respectively for the transmitted marking and spacing telegraph signalimpulses representing a telegraph message, which are separately detectedat the receiving end of the system and applied to the opposing windingsof the channel polar receiving relay causing its operation to repeat themarking and spacing signals. Where it is desired to transmit thevoice-frequency telegraph signals of such systems between widelyseparated points efciently and cheaply, one of the voice-frequencycarrier telegraph terminals may be located at each of those points, anda portion of an existing high frequency transmission system extendingbetween those points, for example, a high frequency channel of a singleside-band (suppressed carrier) telephone system, utilized fortransmitting the telegraph signals between the voice-frequencyterminals. In that case, a group modulator with an associated highfrequency carrier source at the transmitting end of the high frequencychannel would be utilized for stepping up the fundamentalvoice-frequency carrier channels to the higher frequencies required fortransmission over it, and a group demodulator with an associated highfrequency carrier source at the receiving end of the high frequencychannel, for stepping down the received carrier telegraph channels tovoice frequencies for reception.

In certain of the existing high frequency systems which may be used insuch a combination system, the high frequency oscillators associatedwith the group modulator and demodulator in the high frequency channelfor each direction are normally operated unsynchronized. They may,therefore, have instantaneous frequency differences between them attimes which may be sufficient, particularly where the telegraph signalsare applied to the voice-frequency carriers by frequencyv modulation, tointroduce intolerable signal bias variations in the received telegraphchannels.

AAn object of the invention is to reduce signal distortion in carriertelegraph systems.

A more specific object is to reduce to an unobjectionable amount signalbias variations in the telegraph channels of a multichannel voice-fre- 5claims. (01.179-4) quency carrier telegraph system employing frequencymodulation, operated over a high frequency channel employingunsynchronized high frequency carrier sources associated with the groupmodulation and demodulation apparatus at its opposite terminals.

These objects are attained in accordance with the invention by sendingout over the high frequencyV line of such a system, along with thetelegraph signal-modulated carriers, an unmodulated pilot wave ofsuitable frequency, which at the receivingend of the system is combinedin a modulator with the received signal-modulated carriers prior totheir separation and detection, to transform them to desired 10Wfrequencies. The final modulation process effectively cancels out fromthe received signals any variations introduced by deviations between thefrequencies of the unsynchronized high frequency carrier sourcesassociated with the group modulator and demodulator in the highfrequency link.

'I'he various features and objects of the invention will be betterunderstood from the following detailed description when read inconjunction with the accompanying drawings in which:

Figs. 1 to 3 in combination show schematically a two-Way carriertelegraph system employing frequency modulation, embodying theinvention; and

Figs. 4 to 7 show curves used to explain the invention.

Figs. 1 to 3, when placed side by side in that order with Fig. 1 at theleft, show a complete twoway carrier telegraph system providing fourmessage channels in each direction, consisting of a west and an easttwo-Way voice frequency carrier telegraph termina1 and an intermediatetwo-way high frequency transmission link HFL, which may be one two-Wayhigh frequency channel of a multichannel single side-band carriertelephone system, converted to telegraph use.

The high frequency transmission link HFL of the combination system, asshown in Fig. 1, comprises a west-to-east one-way high frequencyrepeating path EA and an east-to-west one-way high frequency repeatingpath WA coupled at their west and east terminals in conjugate relationwith each other and in energy transmitting relation with the westtwo-way line section LW and the east two-way line section LE by means ofthe hybrid repeating coil HCW and the associted line balancing networkNw, and the hybrid repeating coil HCE and associated line balancingnetwork NE, respectively. The path EA includes in its input and outputthe band-pass filters F23 and F24 respectively adapted to pass only aband of frequencies corresponding to the total frequency range of themodulated voice-frequency carriers and pilot wave generated at the Westvoice-frequency carrier telegraph terminal, and the path WA in its inputand output the bandpass filters F25 and F26 respectively adapted to passonly a band `of frequencies corresponding to the total frequency rangeof the modulated voice frequency carriers and pilot wave generated atthe east voice frequency carrier telegraph termi'-` nal in the manner tobe described. The path EA also includes in its input, to the east offilter F23, a group modulator M1 and associated carrier source O11generating the high frequency f1, and in its output, to the West ofband-pass filter F24, a group demodulator D1 with an associated carriersource O12 of corresponding nominal high frequency f1. The path WA alsoincludes in its input, to the West of filter F25, a group modulator M2,with an associated carrier source O12 of the high frequency f2; and inits output to the east of lter F25 a group demcdulator D2 with anassociated carrier source O14 of the nominal high frequency f2.

The transmitting circuits of the two-way voice frequency carriertelegraph terminal at the west and of the combined system, as shown inFig, 1, include four oscillators O1 to O4 normally generating carriercurrents of the voice frequencies 1445, 16115, 1785 and 1955 cycles forthe four transmitting channels I to 4. These oscillators may be of thevacuum tube type illustrated for the oscillator O1 in channel I which isshown in more detail. Thesecarrier currents are respectively modulatedby separate sending circuits associated with the respective oscillators,to impress telegraph signals constituting a separate message on eachcarrier. The sending circuit in channel I comprises the polar telegraphvsending relay SR1 which is controlled by the telegraph key K1 or Vitsequivalent in one subscribers loop. Similarly, the sending circuit ineach of the other three channels comprises one of the polar telegraphsending relays SR2 to SR1 respectively controlled by one of theoperating keys K2 to K4, or theV equivalent, in other respectivesubscribers loops.

When the key K1 in channel I is closed to provide an energizing circuitfrom the associated Y battery to the operating Winding of sending. relaySR1, that relay is operated to its marking contact m1, and when the keyK1 is opened to deenergize the operating Winding, that relay is operatedtoits spacing contact s. The relay connectionsto oscillator O1 are suchthat the capacitance in the tuned circuit of that oscillator is changedby a sucient amount for the two operating conditions of relay SR1k toswing the carrier generated by the oscillator cycles to one side of theVnormal carrier frequency (1445 cycles) for a marking signal and 35cycles on thepother side of the normal carrier frequency for a spacingsignal. Thus, by alternately closing and Opening thekey K1, the carrierWave of nominal frequencyy 1445 cycles in channel I may be frequencymodulated in accordance with a telegraph message to be transmitted.

Similarly, the; frequencies of the carriersV generated, by the similaroscillators O2 to O4, indicated by boxes, in` channels 2 to 4,respectively,

may be. shifted i235 cycles from the nominal values of 1615, 1785 and1955 cycles, respectively, by operation of the, associated sending relaySR2 to VSlis` alternatelyv to their marking andv spacing contacts undercontrol of the telegraph keys K2 to K4 in the associated subscribersloops, to provide frequency-modulated voice-frequency carrier Wavesrepresenting respectively different telegraph messages to betransmitted. The frequency-modulated carriers in the output ofoscillators O1 to O4 in the four channels I to 4 pass respectivelythrough the associated sending band filters F1 to F4, respectivelyadapted to pass the frequency bands 1445i35 cycles, 1615x135 cycles,1785i35 cycles and 1955i35 cycles.

Irl addition to the four carrier oscillators O1 to O4 at the Westterminal, there is provided a Vfifth oscillator O5 which generatesanother voicefrequency of 2380 cycles which passes unmodulated throughthe associated filter F5 selective to its frequency. The outputs of theve filters F1 to F5 are bridge in parallel across the common two-Wayline section LW so that the four frequency-modulated carriers and theunmodulated pilot Wave of a frequency of 2380 cycles are superposed inthat line section and pass thereover to the West terminal of the highfrequency transmission link HFL.

Similarly, at the east end of the system the transmitting circuits ofthe east voice-frequencyA carrier telegraph terminal, as shown in Fig.2, include four oscillators Os to O9 normally generating carriercurrents of the different voice frequencies 595, 765, 935, 1105 cyclesfor the four carrier telegraph channels in the east-to-West direction.The oscillators O1 to Os are adapted to be respectively controlled bythe associated sending circuit comprising sending relay SRs and theassociated subscribers key Ks, the sending relay SR1 and the associatedsubscribers key K7, the sending relay SRs and the associated subscriberskey Ks, or the sending relay SR@ and the associated subscribers key K9in a manner similar to that described for the similar channels at thewest terminal of the system, to swing thev carrier for each channel 35cycles to one side of its normal frequency value for a marking signaland 35 cycles to the other side for a spacing signal. In addition to thefour carrier oscillators O5 to Os at the east terminal, there is a fifthcarrier oscillator O10 generating' a frequency of 2550 cycles which isunmodulated by telegraph signals.. The marking and spacing telegraphsignal outputs of the carrier oscillators Os t0 On and the unmodulatedoutput of the 2550-cycle oscillator 01o are respectively selected by thetransmitting band filters Fe to F111 and are superposed in the two-Wayline section LE over which they pass to the east terminal of the highfrequency transmission line HFL.

The voice-frequency carrier telegraph receiving circuits at the eastterminal of the system, shown in Figs. 2 and 3, include a balancedmodulator M31 which may be of the second order type, such as disclosed,for example, in F.Y A. Cowan Patentk 1,959,459. The modulator M2includes four copper-oxide rectifier units connected ina Wheatstonebridge formation; an output transformer I connected across one diagonalof the bridge, one input circuit, including the bandpass filter F11,passing the frequency range 1375- 2025 cycles corresponding to thefrequency range occupied by both the marking andY spacing signals of allfour voice-frequency carrier telegraph channels. generated at the westterminal, connected by transformer 2 across that samediag-- onal of thebridge, and asecond input circuit, including the narrow band filter F12selective to the frequency 2380 cycles produced by the pilot waveoscillator O at the west terminal, connected by transformer 3 across theother conjugate diagonal of the bridge. The two input circuits ofmodulator M3 are fed in common through transformer 4 from the two-wayline section LE terminating the east end of the high frequency channelHFL with the waves of voice frequency received from that channel, thepass frequency ranges of filters F11 and F12 being such as to preventthe outgoing signal outputs of the transmitting band filters Fs to F1obridged across the line section- LE from feeding into the input of themodulator M3.

Neglecting for the" momentl the effect of any variations produced in thetransmitted signalbearing voice frequency carriers because' ofvariations between the frequencies of fthe unsynchronized high frequencycarrier oscillators in the intermediate high frequency carrier link HFL,the operation of the voice frequency carrier telegraph receivingcircuits at the east terminal is as follows: The fourfrequency-modulated carrier waves of the frequencies 14451-35 cycles,16151-35 cycles, 1785i35 cycles and 1955i35 cycles and the oneunmodulated pilot wave of 2380 cycles received in the line section LEover the high frequency carrier link I-IFL from the west terminal of thesystem will pass through the transformer 4 to the conjugate inputcircuits of modulator M3. The four signal bearing waves will be selectedby fthe band filter F11 and will be impressed by transformer Z on onediagonal of the copper-oxide rectifier bridge of the modulator M3 andwill combine with the 2380-cycle unmodulated pilot wave, selected bylter F12 and impressed by transformer 3 on the other conjugate diagonalof the bridge, to produce the desired output frequencies shown in Table1 to the right of modulator M3. These desired frequencies arerespectively selected and passed on by the narrow receiving channel bandfilters F13, F14, F15 and F16 connected in parallel to the output ofmodulator M3, the undesired products of modulation in modulator M3 beingrejected by those filters. From this point it will be suilcient to tracethe path in one channel only as all perform in a similar manner.

Considering receiving channel I, the current out of the 425-cycle iilterF1a swings back and forth between 4254-35 cycles and 425-35 cycles forthe received marking and spacing signals. This current of varyingfrequency then passes into the current limiter CL1 comprising the twotransformer coupled-amplifying vacuum tube stages 1 and 8, a portion ofthe output of the second stage tube 8 being picked olf through a thirdwinding 9 on output transformer l0, rectiiied by the full-wave rectifierIl and passed through the series resistance l2 in the controlgrid-cathode circuit of tube 8 to provide a varying direct current biason its control grid. The circuit constants of the current limiter CL1and those of the similar current limiters in the other receivingchannels at both terminals of the system are selected such as to producea current input-current output characteristic resembling that of Fig. 4.It will be observed that for a wide range of input levels, the output isnearly constant. The purpose of this limiter circuit is to provide atits `output current of closely constant amplitude, regardless of thefrequency variation of its current input and regardless of the ratherwide swings in amplitude which may be caused by line equivalentvariations. There then will be impressed on the marking discriminatorD1v11 and the spacing discriminator Der having their inputs connected inparallel to the output of the current limiter CL1', a current ofconstant amplitude and of a frequency whichv may be steady or variable,depending on whether the channel is standing idle or transmittingsignals. f

The discriminators DM1 and Ds1 consist merely; of highand low-pass ltersor tuned circuits, respectively, of simple design. Their characteristicsare shown in Fig. 5 with that of the recelving channel band filter F13,drawn in dotted lines to show the band limits. When the received carriercurrent is at the marking frequency fm, the loss through the markingdiscriminator Divn is low and the loss through the spacing discriminatorDsl is high. Therefore, the marking current out of the discriminator DM1is at a maximum and the spacing current out of the spacing discriminatorDs1 is at a minimum. As lthe frequency is shifted from fm to fs, theloss through the discriminator DM1 increases and that through thediscriminator Ds1 decreases. When the carrier reaches fs, the currentout of the discriminator DS1 has reached a maximum and that out of thediscriminator DM1 has reached a minimum. It will be noted that themodulation process in Ms has turned over the frequency deviation. Themarking frequency in channel l started out from the west terminal as1955-35 cycles and became 4254-35 cycles in the output of modulator M3.

The currents out of the discriminators DM1 and Ds1 are ampliiied andrectified in the marking detector MD1 and the spacing detector SD1,respectively, and the result is a push-pull current in the marking andspacing windings of the polar receiving relay RR1, respectivelyconnected to the outputs of those detectors. Typical marking and spacingwinding currents in the receiving relay RR1 are shown in the curves ofFig. 6.

The push-pull currents in the lwindings of the relay RR1, produced bythe detected marking and spacing current-s, cause the armature of thatrelay to move back and forth between its marking contact m and itsspacing contact s, which repeats the telegraph signals to thesubscribers receiving loop connected to those contacts causing theoperation of the printer, sounder or other receiving or recording devicein that loop in accordance with the repeated signals. In receivingchannels 2, 3 and 4, the receiving apparatus is similar to the receivingapparatus in channel I as indicated by the use of the sameidentification characters but with suitable subscripts corresponding tothe number of the channel, and operates in response to the marking andspacing signals in the output of modulator M3 of the frequencies 595i35cycles, 765i35 cycles and 935i35 cycles, respectively, in a mannersimilar to that which has just been described for channel I to causeoperation of the channel receiving relays RRZ to RR4 to repeat thesesignals to the subscribers receiving loops respectively connected to thechannels.

Similarly, the voice-frequency telegraph receiving circuits at the westterminal of the system, as shown in Fig. 1, include a balanced secondorder modulator M4 of the copper-oxide rectifier bridge type, similar tothe modulator M3 at the east terminal, having its conjugate inputbranches respectively including a band-pass filter F17 and the narrowband-pass filter F1a fed in common through transformer 5 from the twowayline section LW terminating the west end of the high frequencytransmission link I-IFL, and its' single output circuitcoupled .byoutput transformer E .in .parallelto the inputsof the'four receivingband filters F19 to F22 in the receiving carrier telegraph channels l toIl. 'The four frequency-modulated carrier Waves of the frequencies595135 cycles, 765i35 cycles, 9351-.35 cycles and 1105135 cyclesgenerated by the sending circuits at the east terminal and theaccompanying unmodulated pilot Wave of the frequency 2550 cyclesgenerated at that terminal pass from the two-Way line section LW throughthe input transformer f to the modulator Mi. The four signal bearingwaves are selected out 'by the band filter F11 passing the frequencyrange 525 to 11'75 cycles in one input branch, and are combined in themodulator 'M4 with the 2550-cyclerpilot current selected by the lter Figin the other conjugate input branch, to produce the four desired outputfrequencies in the output of the modulator, shown in Table 2 to the leftof that modulator. These desiredfrequencies pass through outputtransformer 6 and are respectivelyselected by the receiving band filtersF19 to F22 in the four receiving channels, the undesired products ofmodulation in the output of the modulator M4 'being rejected by theselters.

In receiving channel I at the West terminal, the current output offilter F19 swings back and forth between 1445-i-35 and 1445-35 cycleswith the marking and spacing signals. This current of varying frequencythen passes through the individual current limiter CL5 for channel l,which provides an output current of substantially constant amplituderegardless of the frequency variation and regardless of the .wide swingsin amplitude in the current input caused by line equivalent variations.The output of the current limiter CLs is impressed on theparallelconnected inputs of the marking and spacing discriminators Divisand Us5, similar to the corresponding discriminators in each receivingchannel at the east terminal just described and operating in similarmanner. The currents out of the discriminators D215 and Dss arerespectively amplified and detected in the marking detector MD5 and thespacing detector SDs, and the resulting detected currents are applied inpush-pull to the marking and spacing windings of the polar receivingrelay RRs to cause alternate operation of its armature to its markingand spacing contacts to repeat the telegraph signals to the subscribersreceiving loop associated with that channel.

Similarly, the frequency-modulated carrier outputs of the filters F20 toF22 in the telegraph receiving -channels 2 to A at the West terminal, ofthe frequencies ,1615x351 cycles, 1785i35 cycles and 1955i35 cycles, arerespectively operated -on by the current limiter, marking and spacingdiscriminators, and the marking and spacing detectors of thecorresponding channels in a manner similar to that which has beendescribed for the similar elements in channel l, to provide respectivealternate operation of the channel receiving relays RRG to RRS to theirmarking and spacing contacts, to repeat the marking and spacingtelegraph signals for those channels to the connected subscribersreceiving loop to operate a printer, sounder or other receiving orrecording device.

In the above description of the passage of the frequency-modulatedcarrier waves and of the unmodulated pilot wave from the transmittingapparatus at the West voice frequencyl carrier telegraph 'terminal tothe voice frequency telegraph receiving apparatus at the east terminal,and vice versa, the action of the high frequency apparatus in theintermediate high frequency transmission link HFL, not previouslydescribed in detail, is as follows.

The easterly directed voice-frequency carrier channels and theaccompanying unmodulated pilot wave generated by the voice-frequencycarrier telegraph transmitting apparatus at the west terminal passinginto the two-Way line sections LW at that terminal, will be impressed bythe hybrid coil HCW on the input of the West-toeast repeating path EA inthe link I-lFL in which they will be selected by the band-pass filterF23 having a pass range which includes all of those frequencies butexcludes those of the westerly directed voice-frequency channels andpilot Wave. The selected voice frequency-modulated carrier output of thefilter F23 in the path EA will he impressed on the modulator M1 in Whichit Will be combined with the high frequency carrier of nominal value f1supplied from the associated oscillator O11, to step up thevoice-frequency carrier channels and the pilot Wave, to frequenciessuitable for transmission over that path. At the east terminal 0f thehigh frequency path EA, the received carrier channels of stepped-upfrequencies are combined in the demodulator D1 with a high frequencycarrier Wave of the same nominal frequency f1 supplied to thedemodulator from the associated oscillator O12, to step the carrierchannels down again to their original frequencies Which are selected bythe band-pass filter F24 in the output of the path EA, having the samepass range as the iilter F23, and are impressed by the hybrid coil HCEon the two- Way line section LE. The received voice-frequency channelsand pilot Wave will pass from the line section LE to the voice-frequencycarrier telegraph receiving circuits of the east terminal describedabove.

Similarly, the Westerly directed voice-frequency carrier channels andthe unmodulated pilot Wave generated in the voice-frequency carriertelegraph transmitting apparatus at the east terminal of the system ofFig. l, passing into the tivoway line section LE will be impressed bythe hybrid coil HCE on the east-to-West repeating path WA of theintermediate high frequency link HEL and will be selected by theband-pass filter F25 having a suitable frequency pass range. Theselected frequencies in the output of the lter F25 will be combined inmodulator M2 in the input of the path WA with the high frequency carrierof nominal frequency fz supplied from theiassociated carrie;` oscillatorO13, to step-up all of the voice-frequency channels and the pilot waveto high frequencies suitable for transmission over that path, and at thewest terminal of the high frequency link HFL the received Waves Will becombined in the demodulator D2 with the high frequency carrier ofnominal frequency f2 supplied from the associated carrier oscillator 012to step them down again to their original voice frequencies. Thesefrequencies Will be selected by the band-pass filter F25 of suitabletransmission frequency range, and impressed by the hybrid coil HCW onthe `two-way line section LW from which they will pass to thevoice-frequency carrier telegraph receiving circuits of the westterminal Adescribed above.

Unless the high frequency carrier Sources associated with 'the groupmodulator and the group demodulator V.in the vrepeating path fOr @30h-di- ',rection in `thehigh frequency link HFL are definitelysynchronized over the line, their frequencies are never exactly alike,and in the particular high frequency channel with which thevoicefrequency carrier telegraph channels of the invention were actuallycombined, it was foundv that they may differ at times by as much as 15cycles. It has been found by experiment that such a 15-cycle deviationbetween the high fre'- quency carrier oscillators with afrequency-modulation voice frequency carrier telegraph system such asillustrated in Figs. 1 to 3, but not -including the arrangements of thepresent invention, would result in a signal bias variation of about 20per cent in the telegraph receiving channels. The characteristics fromwhich this was deduced is shown in Fig. 7. 'Ihe curve of that figureshows the percentage signal bias variation produced in a telegraphreceiving channel of the 'system for different values of frequencydeviation in cycles between the high frequency carrier oscillatorsassociated with the group modulator and the group demodulator at theinput and output, respectively, of the high frequency channel feedingit.

Let us assume that oscillator O11 associated with the group modulator M1in the west-to-east side of the high frequency link HFL in the system ofFig. 1, is at the correctfrequency, and that the oscillator O12associated with the demodulator D1 in the output of that side is l5cycles lower. The easterly directed channel 4 current at frequency1445i35 cycles, after passing through the demodulator D1 at the eastterminal of the high frequency channel would, in a conventionalfrequency modulation system, have a frequency of 1445i35-15 cycles underthe assumed condition of oscillator O12 being 15 cycles low. By the timethis signal reached the receiving relay RRi in receiving channel I atthe east terminal, it will have a light bias of about 20 per cent, asindicated by the curve of Fig. '7, above the point -15 on the frequencyaxis. But in the circuit illustrated in Figs. 1 to 3, a pilot wave ofthe frequency 2380 cycles has been transmitted along with the channelfrequencies and this also emerges from the output of the demodulator Di15 cycles low. When these two currents are combined in the modulator M3in the common receiving circuit at the east terminal of the system, acombination of this type ensues:

(2380-15)-[(144515);35 cyclesl:

935i35 cycles (l) which is the same frequency as would be obtained whenthe frequencies from the oscillators O11 and O12 were considered to beexactly the same. Thus, the frequency variations introduced in theintermediate high frequency line have been effectively cancelled out inthe final modulation process. The same would be truein the case of eachof the other channels. Thus the scheme of the invention may be used tomake the received frequency in any frequency-modulated carrier telegraphchannel independent of any frequency variation between separate andunsynchronized carrier oscillators associated with the modulator anddemodulator in any intermediate high frequency carrier link.

Various modifications of the circuits illustrated and described whichare within the spirit and scope of the invention will occur to personsskilled in the art.

What is claimed is:

1. In combination, a group of low frequency carrier communicationchannels and a separate pilot channel, means at a transmitting point forcombining each of said channels with a high frequency carrier fortransmission, means ata receiving point for combining the received highfrequency waves with a carrier of nominally the same high frequency asthe high frequency carrier at the transmitting point, to demodulate thereceived communication channels and the received pilot channel, andmodulating means for combining each of the demodulated carrier channelswith the demodulated pilot channel to compensate for any frequencydifferences between the high frequency carriers at the transmittingl andreceiving points tending to distort the transmitted carriercommunication channels.

2. In combination in a carrier signaling sys-l tem, a high frequencytransmission line, a plurality of sources respectively generatingcarrier waves of different low frequencies, another source generating apilot wave of predetermined frequency, means to modulate each of saidcarrier waves with a diiferentmessage, means to impress the modulatedcarrier waves and the unmodulated pilot wave on the input o'f said line,a group modulator with an associated high frequency carrier source inthe input of said line for stepping up the frequencies of all of theimpressed waves to those suitable for transmission over that line, agroup demodulator withr an associated high frequency carrier source ofthe same nominal frequency as the first high frequency carrier source,in the output of said line for stepping down the received waves to theiroriginal frequency values, a second modulator fed from the output ofsaid line, for combining each of the applied low frequency-modulatedcarriers with the received unmodulated pilot wave to transform theformer to desired lower frequencies, and to eliminate therefrom anyvariations therein caused by any frequency differences between the highfrequency carrier sources associated with the group modulator and groupdemodulator of the high frequency line, and means to separate theresulting modulated carrier waves and to separately detect the messagecomponents therein.

3. In a carrier wave signalingsystem including said link from the lowfrequency carrier signal transmitting circuits at each terminal to highfrequencies suitable for transmission over that link, and for steppingdown the high frequency carrier signals received at each terminal oversaid link to their original frequency values before supplying them tothe low frequency carrier signal receiving circuits at that terminal,the low frequency carrier signal receiving circuits at each terminalincluding other modulating means for combining each of the received lowfrequency carrier signals with a carrier Wave of suitable frequency totransform the former to desired low frequency values before they areseparated and the signal components detected therefrom, and means toreduce distortion of the detected signal components due torelativeinstantaneous frequency differences between the high frequencycarrier sources in the intermediate high frequency transmission linkcomprising means for supplying said carrier waves of said suitablefrequency comfi bined with each receivedlow frequency 'carrier signalsinthe low frequency carrier signal"A rec'eving circuit at each terminal,over said high frequency transmission link from the other terminalsothat they are subjected to the same frequency variations in that link asthe transmitted carrier signals.

4. In combination in a multiplex carrier telegraph system, means forproducing a plurality of different voice frequency carriers respectivelyfrequency modulated with a different telegraph message, a high frequencytransmission line supplied With said modulated carriers and including inits input modulating means with an associated high frequency carriersource, for stepping up the supplied Waves as a group to the highfrequencies suitable for transmission thereover, and in its outputdemodulating means with an associated high frequency carrier source forstepping down theV Waves received over the line as a group to theiroriginal frequencies, another modulating means for combining themodulated carriers of low' frequencies received over said line with acarrier Wave of suitable frequency to transform the former to suitablefrequency values, means for separating lthe resulting modulated carriersand separately detecting the telegraph message components therefrom, andmeans to reduce distortion in the detected message components resultingfrom fo'rtuitous frequency differences between the high frequencycarrier sources associated with the group modulating and demodulatingmeans in said high frequency line, comprsng means for Supplying saidcarrier Wave of suitable frequency combined with the modulated carriersin saidother modulating means, -over said high frequency line so that itis subjected to the same frequency variations as the modulated carrierstransmitted thereover.

5. The combination of claim 4 in Which each of said plurality ofdifferent4 voice-frequency carriers frequency modulated with a differentmessage is obtained by changing the tuning of a differentVoice-frequency carrier oscillator under control of associated keyingmeans so that the generated carrier is swung a given number of cycles toone side of the normal carrier frequency to produce a marking signalandl the same amount to the other side of the normal carrier frequencyto produce a spacing signal, the receiving circuit of said systemcomprising separate nlters for respectively `selectingboth the markingsignal component and the spacing signal component in each modulatedcarrier VWave in the output of said other modulation means at thereceiving terminal, separate current Alimiters for respectivelyproducing from the current output of each of said filters a current ofsubstantially constant amplitude, regardless of frequency and amplitudevariations therein, other ltering means for separating the marking andspacing signal impulses in the resulting Wave, detectors for separatelydetecting said marking and spacing signal impulses and a receivingtelegraph relay having opposing windings to which the detected markingand spacing impulses are respectively applied to cause the operation ofsaid relay to repeat the marking and spacing signals of the message.

WALTER A. PHELPS.

